In optical communication using an optical fiber, multi-mode communication has been known which performs information communication by superimposing information on light of an LP01 mode (basic mode) and superimposing information on light of an LP mode of a higher order than that of the basic mode such as an LP11 mode. In the multi-mode communication, light of a plurality of LP modes propagating through one core is demultiplexed into a plurality of optical fibers, and light of mutually-different LP modes propagating through a plurality of optical fibers is multiplexed into one optical fiber.
In Non Patent Literature 1 described below, it has been described that, in a case where light of a same LP mode is propagated through one set of optical fibers, light propagating through the optical fibers is multiplexed as mutually-different LP modes by using a mode converter or the like.
In addition, a mode multiplexer/demultiplexer disclosed in Patent Literature 1 described below includes two waveguides. Between the waveguides, one waveguide propagates light of a specific LP mode, and the other waveguide propagates light of a specific LP mode and light of an LP mode other than the specific LP mode. In a case where light of a same specific LP mode propagates through such waveguides, a propagation constant of one waveguide for light of the specific LP mode and a propagation constant of the other waveguide for light of the other LP mode are configured to coincide with each other. According to such a configuration, the light of the specific LP mode of one waveguide is multiplexed as light of the other LP mode of the other waveguide. The Light of the specific LP mode is originally propagated through the other waveguide, and, as a result, light of an LP mode other than the light of the specific LP mode is propagated through the other waveguide. In this way, in a case where light of a same LP mode is propagated through a plurality of optical waveguides without using a mode converter, each light can be propagated as light of mutually-different modes through one waveguide.    [Non Patent Literature 1] An Li et al., “Low-Loss Fused Mode Coupler for Few-Mode Transmission”, OFC, OTu3G4 (2013)    [Patent Literature 1] JP-A-2013-37017